Ultraporous and microporous membranes and method of making membranes

ABSTRACT

Ultraporous and microporous polymer membranes cast from metastable dispersions are significantly improved by limiting the time of environmental exposure to less than about 0.5, preferably less than 0.25 seconds, between casting about 10° to about 20° C. lower than the usual prior art values. The resulting membranes have far less debris entrained in the membrane, far more consistent and uniform pore sizes, a substantially greater number of skin pores, and greatly increased flow rates for any given pore diameter.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to ultraporous and microporous membraneswhich are useful in materials separations, by filtration, dialysis, andthe like, and as supports and containment media for materials, andrelated uses. In particular, it relates to highly asymmetric, integralmembranes with a skin and a porous sub-structure or support region.

2. Description of the Prior Art

A wide diversity of polymer membranes are known, and have attained wideapplicability in diverse uses. Such membranes are characterized by avariety of properties and characteristics, and the selection of amembranes for a particular use is generally a function of the propertiesrequired or desired.

The most characteristic property of concern for most applications is theeffective controlling pore diameter, which defines what materials maypass through the membrane, and which are retained. Ultraporous membranesare generally those with an effective controlling pore diameter of lessthan about 0.050 micrometers (or sometimes considered to be less thanabout 0.025 micrometers), down to as 0.005 micrometers, which is therealm of the size of a molecule of, for example, simple sugars and thelike. Microporous membranes are those with effective limiting porediameters of greater than ultraporous, normally greater than about 0.050micrometers, up to about 1 micrometer, or occasionally more.

As used in the present application, the term "pore diameter" is employedto represent the span across skin pores or controlling pores of amembrane. It is not intended to suggest that all pores are circular and,indeed, most are not, as those of ordinary skill in the art willunderstand and as FIG. 2 illustrates.

Smaller pore membranes extend into the reverse osmosis region and belowthat into the gas separation region. Reverse osmosis membranes are usedfor ionic separations, under high applied pressure differentials,sufficient to overcome osmotic pressure, and are sometimes said to bedependent on a mechanism which is often characterized as intermoleculardissolution. Such membranes have a dense, non-porous surface skin, anddo not function by effects dependent on seive-like characteristics. As adistinguishing characteristic, reverse osmosis is, in material part,dependent on the osmolarity of a solution as a determinant of theseparatory characteristics of the reverse osmosis operation, whileultraporous and microporous membranes pass or retain materialspredominantly on the basis of their size, at applied pressuredifferential which are commonly far less, often an order of magnitudeless, than reverse osmosis operations, and are ordinarily considered tobe substantially different in kind. Gas separation membranes operate ona molecular scale and fractionate gas mixtures based on size andabsorption/desorption characteristics.

An important property of porous membranes is their permeability to flow.In the majority of applications, it is commonly desirable to processeffectively the greatest volume of a feed material in the least amountof time. All other things being equal, the higher the flow rate offiltrate or related materials through the membrane, the higher theefficiency and economy of the procedure.

It has long been known that flow rates are proportional to porediameters and pore population. Taken together, these define an effectivearea through which fluids may pass. In practice, the relationship isordinarily very approximate and highly variable.

Membranes may have a skin or may be skinless, i.e. with an isotropicstructure from one face to the other. If a cast liquid film of adequatepolymer concentration is quenched in a strong non-solvent, as withpolysulfone solutions (or dispersions) quenched in water, the result isa "skinned" membrane, i.e., one with considerably smaller pores on the"skin" side than on the opposite side. If the quench liquid is a weaknon-solvent, e.g., by adding solvent to the water, a more open skin andultimately a skinless membrane can be produced.

When a skin is present, as generally in the case with gas separation,reverse osmosis, and ultrafiltration membranes and sometimes withmicroporous membranes, it is most often a dense film of polymer materialwith very small pores that extend into a support region of larger pores.If the pores are large enough, they can be observed by electron scanningmicroscopy, and this is true in the microporous range. However, becauseof the limitations of SEM techniques pores may not always be directlyobservable at diameters of less than about 0.050 micrometers, but theirpresence can be confirmed by the retentivity characteristics of themembrane.

Membranes can have different structures, generally determined by thetechnique by which it is synthesized. Examples include fibrous,granular, cellular, and spinodal, and they may be symmetrical, orasymmetric, isotropic or anisotropic (i.e., graded pore density).

Fibrous microstructure is most commonly associated with biaxialstretching of films of polymers. This is commonly employed, for example,in the production of porous membranes of polytetrafluoroethylene(TEFLON)®, in the microporous membranes commercially available asGORETEX®, among others. It is inherent in the nature of the process thatthe result is a skinless, symmetrical membrane.

Granular microstructure can be characteristic of membranes formed by theprecipitation of polymer from certain formulations by a nucleation andgrowth mechanism. Globules or granules of precipitated polymer form andgrow, and fuse with other such globules at their points of contact,leaving voids in the interstices which contribute the porosity of thegranular mass. Such structures frequently contain "macrovoids" or"finger voids" in regions adjacent to skin imperfections which allow thequench liquid to penetrate the interior. The voids consequently are alsoskinned and lead to reduced membrane permeability. This occurs mostcommonly in ultraporous and reverse osmosis membranes. The techniquesfor the formation of such membranes are illustrated by Michaels, U.S.Pat. No. 3,615,024. The granular microstructure and characteristic"macrovoids" are illustrated by the photomicrographs shown in Wang, U.S.Pat. No. 3,988,245.

Cellular pore structures which are honeycombed or sponge-like inappearance, are dependent presumably on a precipitation rate that isslower than with granular structures containing macrovoids. They can beskinned or unskinned. The latter structure generally is formed when theprecipitation agent is moisture in the air (no liquid quenching duringthe curing process). A network of thin struts creates the system ofcontiguous polyhedral shaped cells. Liquid quenched membranes of thistype are often associated with a dense or ultraporous skin.

Spinodal microstructure, as mentioned earlier, occurs when the polymeris precipitated by a spinodal decomposition mechanism, characterized bythe formation of two separate liquid phases, one polymer rich and theother polymer poor, under conditions wherein each phase is continuousand dispersed in a characteristic pattern at the point at which thepolymer precipitation occurs. Depending on the specific characteristicsof the technique for attaining the spinodal decomposition mechanism, theresulting membrane may be, on the one hand, skinless, symmetrical, anduniform throughout, or skinned, asymmetric, and non-isotropic.

In the present application, the term spinodal structure is intended tomean the characteristic structure attained when a membrane isprecipitated by spinodal decomposition, and to reflect the featuresillustrated in FIG. 1 and, in different scale, FIG. 8 which illustrate,by SEM photomicroscopy the remaining structure when the two,intertangled and intermixed continuous phases of spinodal decompositionare achieved. As those of ordinary skill in the art will understand, thespinodal structure represents one of the two continuous phases formed bythe precipitated polymer, the other being the void volume within thestructure.

The skinless symmetrical variety may be formed by thermal quenchtechniques or by solvent evaporation techniques. Thermal quenchingtechniques are illustrated by Castro, U.S. Pat. No. 4,247,498.

Skinned membranes with a highly asymmetric support structure are shownin Wrasidlo, U.S. Pat. No. 4,629,563, and Wrasidlo, U.S. Pat. No.4,774,039. These membranes are formed by spinodal decomposition inducedby solvent extraction from a cast metastable dispersion of two liquidphases, one polymer rich and the other polymer poor, in a liquid quenchbath.

All the various techniques involved, and the membranes produced, haveachieved a measure of commercial success. The spinodal microstructure,however, has often been preferred in a number of applications. As ageneral rule, the structure affords good mechanical properties,including tensile strength, elongation at break, and the like, thelowest hydraulic resistance to flow of any of the known microstructures,and offers opportunities to take advantage of the internal structure ofthe support as a depth filter, as a containment medium for materials,and other like advantages. As is well known to the art, the skinless,symmetric varieties have rather different uses that the skinned, highlyasymmetric membranes of Wrasidlo.

In the dispersion casting technique of Wrasidlo, a number ofdisadvantages have been encountered. These include the following:

When the polymer is precipitated from the dispersion, there are frequentoccurrences of small discontinuities. The reason for this is not fullyunderstood, but the result is the formation, within the microstructureof the membrane support, substantial number, and at time vast numbers oftiny polymer spheres. These discrete spheres are difficult to remove bywashing, and substantial numbers may remain in the membrane. This ishighly undesirable, in most uses of the membranes, since there are fewapplications where the introduction of these spheres into a filtrate isacceptable. See the spheres illustrated in FIG. 7, which represent asevere case, after normal washing of the membrane.

The procedure for the formation of the membranes taught by Wrasidlo hasover time proved to be excessively variable in the controlling porediameter, flow rate for a given pore diameter, and in the occurrence ofmacro flaws in the integrity of the skin, leading to the loss of anunacceptable proportion of the membranes to a failure to satisfynecessary quality control standards. Quality control rejection of suchmembranes often has been substantial.

Some physical properties, including tensile strength and elongation atbreak, are often lower than desirable, and lower than required for theintegrity of some otherwise desirable uses of these membranes.

These membranes are often employed in critical applications in theelectronics industry, food processing, processing of biologicalmaterials, as sterilizing filters, and the like. Deficiencies in meetingthe quality control requirements of such sensitive fields of use arequite unacceptable.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to improve the technology bywhich skinned, asymmetric ultraporous and microporous membranes having asupport structure with an asymmetric spinodal structure are produced,and to provide such membranes with improved properties andcharacteristics, and to satisfy the highest standards of quality controland product integrity.

The present invention is intended as, and has as its primary object, animprovement on the process and product of Wrasidlo, cited above. Thedisclosure of the Wrasidlo patents is incorporated herein by reference.

In one aspect, the present invention provides an improved method formaking the ultraporous and microporous membranes, wherein interaction ofthe cast dispersion with the atmosphere prior to solvent extraction in aquench bath is limited to less than 0.5 seconds, and preferably lessthan 0.25 seconds, and where the casting temperature of the dispersionis materially reduced.

In another aspect of the present invention, ultraporous and microporousmembranes are produced which are substantially free of polymer spheresentrained in the support, have increased tensile strength and elongationat break, a materially reduced standard deviation in controlling porediameter, and an increased population of skin pores, resulting in anddemonstrated by exceptionally high flow rates in relation to thecontrolling pore diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a reproduction of an SEM photomicrograph showing thecharacteristic asymmetric spinodal structure of the support region ofmembranes of the present invention, in a cross section of a fractureface at an enlargement of 650×.

FIG. 2 is a reproduction of an SEM photomicrograph showing thecharacteristic skin pores of a membrane of the present invention at anenlargement of 3,000×.

FIG. 3 is a graph showing the relationship of bubble point and flow formembranes of the present invention compared to comparable, historicalvalues for membranes of the prior art, represented by the Wrasidlopatents cited above.

FIG. 4 is a graph showing the relationship of increase in flow rate ofthe membranes of the invention over the corresponding historical valuesof production of Wrasidlo membranes of comparable bubble point.

FIG. 5 is a graph of bubble point covariance for membranes of thepresent invention compared to historical, corresponding measurements ofmembranes of the prior art, represented by the Wrasidlo patents citedabove.

FIG. 6 is a graph of flow rate covariance for membranes of the presentinvention compared to historical, corresponding measurements ofmembranes of the prior art, represented by the Wrasidlo patents citedabove.

FIG. 7 is reproduction of an SEM photomicrograph showing a membrane madein accordance with Example II of Wrasidlo, showing the high populationof polymer spheres, in a section of the support region at an enlargementof 1,800×. A spinodal structure is apparent.

FIG. 8 is a reproduction of an SEM photomicrograph showing a membranemade in accordance with the present invention, showing the very lowpopulation of polymer spheres, in a section of the support region at anenlargement of 1,800×. The spinodal structure of the membrane is shownin detail.

DETAILED DESCRIPTION

The present invention is directed to improved membranes, and to theimproved method for making such membranes. In the following discussion,a characterization of the method for making membranes is described andcharacterized initially.

In the method of the present invention, the process of the Wrasidlopatents, discussed and cited hereinabove, is the starting point of thework, and the relevant disclosure of the Wrasidlo method is accordinglyincorporated by reference.

In present method, the major elements which differ from Wrasidlo arefound in the limitation of the interaction of the cast dispersion withthe atmosphere prior to solvent extraction in a quench bath to less than0.5 seconds, and preferably less than 0.25 seconds, and the employmentof a casting temperature of the dispersion materially less than thosecommonly employed in the usual techniques, generally on the order ofabout 6° to 14° C. or more. Other process parameters may be subject toadjustment in relation to these parameters to assure the maintenance ofthe requisite properties in the membrane produced, but these aregenerally minor in degree and are reactive to and compensatory for theprimary changes in the operating conditions and steps.

It has long been known that the cast film of the metastable dispersionin the Wrasidlo technology interacts with the atmosphere, and that theproperties of the membrane, and particularly the skin pores of themembrane, are sensitive to variations in temperature and humidity,velocity and direction of air flow, and perhaps others. Controllingthese parameters has received a considerable level of attention and,indeed, some have taken steps intended to take advantage of theseinteractions. See Fuji Photo Film Co. Ltd., G.B. 2, 199,786 A, whereinthe exposure and dwell time in the atmosphere and the humidity areincreased to attempt to achieve certain benefits. The conventionalthinking has been that moisture in the atmosphere initiates therequisite pores at the immediate film/air interface which have diametersproportional to the water vapor concentration and exposure times.

The view in the art for liquid quenched hydrophobic membrane productionhas at least in part been influenced by the limitations of the commonlyemployed equipment used to cast and quench such membranes. In largemeasure, such equipment has been designed and built on the basis thatrelatively long exposure times in the atmosphere are generallybeneficial. It has been common to employ residence times of atmosphericexposure greater than 1 second, and often longer than 5 seconds. Mostequipment cannot attain dwell times between the casting operation andthe quench bath of less than 1 second without specific modifications.Since such modifications run counter to the conventional wisdom in theart, there has never been any incentive to do so.

It has now been found that residence times of materially less than 1second, employed with a materially reduced casting temperature, are ofquite surprising and unexpected benefit. As discussed in detail below,the membrane product is quite substantially improved in a number ofproperties and parameters.

As Wrasidlo has previously pointed out, all the parameters of thecasting procedure and the conditions of operation are mutuallyinterdependent. A change of one parameter will require a correlatingchange in at least one other parameter. If the environmental residencetime is reduced, as required in the present invention, to less than 0.5seconds, and preferably less than 0.25 seconds, for a castingformulation and conditions which had been developed for a longerexposure time, of, say, 1 to 5 seconds, an unsuitable membrane will beobtained if no other parameters are adjusted to balance the effect onthe system of the rapid passage into the quench liquid.

It has also been known in the art that for any given dispersion and anygiven set of casting conditions, there is a relatively narrow castingtemperature variance which will be effective. This is generallymonitored directly, and correlated with the optical density of themetastable dispersion, in order to produce the desired membrane. If thetemperature of the casting dope is too high or too low, unsuitablemembrane or even no membrane at all will be produced. In the context ofthe present invention, the appropriate temperature will be lower thanthat for the same dispersion when cast at longer environmental dwelltimes, but it is not possible to define the precise temperature withoutworking trials of the system and confirmation of the results throughanalytical techniques common to the art. As a general rule, thenecessary casting temperature will be on the order of 6° to 14° C. ormore, often on the order of about 10° to 12° C. below the castingtemperature appropriate to 1 second environmental dwell time.

The lower casting temperature is an important benefit to the process. Ithas been learned that lower temperatures make the system less variableand less vulnerable to variation in substantially all parameters. As thetemperature gets closer to ambient temperatures, as one example,distortions of the casting equipment through thermal expansion and likeeffects are reduced, and it has proved simpler and more reliable tomaintain working tolerances. This in turn provides better control overthe casting and quenching operations, so that quality of the product iseasier to establish and maintain. Utility costs of the operation arereduced and, in new installations, simpler temperature control equipmentmay prove effective.

It is believed, although there is no wish to be bound thereby, thatreduced casting temperature plays a direct role in the reduction ofpolymer spheres entrained in the membrane as an artifact of smallproportions of a separately dispersed phase in the casting dispersion.See FIG. 7, which shows such polymer spheres in a prior art membrane. Itappears that the spinodal decomposition mechanism operates moreuniformly and exclusively at the lower temperatures employed in thepresent invention, resulting in substantially no incidence of suchpolymer spheres. It is possible that this result is partly or whollyattributable to some other factor involved in the system, of course, butit remains the case that the high temperature casting of the prior artconsistently produced such polymer spheres, and few and often none areobserved in the procedure of the present invention.

In practice, the method of the present invention involves the essentialsteps of mixing a polymer, a solvent, and a non-solvent to produce ametastable liquid-liquid dispersion consisting of a polymer-rich phaseand a solvent-rich (polymer-poor) phase within the binodal or spinodalat a casting temperature, casting the dispersion into a thin film at thecasting temperature, passing the cast layer within a time of less than0.5 seconds at the casting temperature into a solvent extraction quenchbath of non-solvent quench liquid in which the solvent is freelymiscible and in which the polymer is substantially insoluble, andeffecting precipitation of the polymer by spinodal decomposition, andrecovering the membrane from the quench bath.

As noted in the Wrasidlo patents, cited above and incorporated herein byreference, a substantial number of polymers, solvents, non-solvents, andquench liquids have been employed, and have been formulated into castingdispersions suitable for casting membranes with a wide spectrum of porediameters. All of these are contemplated in the present invention. Forconvenience, the present invention is discussed in the most common ofthese systems, where the polymer is a polysulfone, the solvent isdimethylformamide, the non-solvent diluent is t-amyl alcohol, and thequench liquid is water.

There are several standard pore diameters which have achieved commoncommercial acceptance for such polyysulfone membranes. These includemolecular weight cutoff values of 10,000 and 100,000 Daltons, and porediameters of 0.1, 0.2 and 0.45 micrometers. The basis for casting thecommercially available membranes according to Wrasidlo is shown in TableI:

                  TABLE I                                                         ______________________________________                                        Pore Diameter                                                                           10K      100K    0.1μ                                                                              0.2μ                                                                             0.45μ                              Polysulfone                                                                             14-16    12-14   10-12  10-12 10-12                                 DMF       80-82    74-76   72-74  72-74 72-74                                 t-Amyl alcohol                                                                          3-5      11-13   14-16  14-16 14-16                                 O. D.     .08-.10  .10-.12 .17-.22                                                                              .20-.32                                                                             .32-.40                               Casting Temp.                                                                           49-52    49-52   49-52  49-52 49-52                                 Time to Quench                                                                          1        1       1      1     1                                     ______________________________________                                         Notes:                                                                        The polysulfone is Amoco Udell P3500.                                         All proportions are in weight percent.                                        O. D. is optical density of the dispersion.                                   Casting temperature is given in °C.                                    Time to quench is the dwell time in seconds between the doctor blade and      the quench bath.                                                         

The corresponding parameters for the same membranes cast in accordancewith the present invention are shown in Table II:

                  TABLE II                                                        ______________________________________                                        Pore Diameter                                                                           10K      100K    0.1μ                                                                              0.2μ                                                                             0.45μ                              Polysulfone                                                                             14-16    12-14   10-12  10-12 10-12                                 DMF       80-82    74-76   73-75  72-74 72-74                                 t-Amyl alcohol                                                                          3-5      11-13   14-16  14-16 14-16                                 O. D.     .08-.10  .10-.12 .24-.25                                                                              .33-.40                                                                             .60-.80                               Casting Temp.                                                                           35-43    35-43   35-40  35-39 35-39                                 Time to Quench                                                                          0.1      0.1     0.1    0.1   0.1                                   ______________________________________                                         Notes:                                                                        The polysulfone is Amoco Udell P3500.                                         All proportions are in weight percent.                                        O. D. is optical density of the dispersion.                                   Casting temperature is given in °C.                                    Time to quench is the dwell time in seconds between the doctor blade and      the quench bath.                                                         

As those of ordinary skill in the art will readily recognize, theresidence time between the casting operation and the quench bath isreduced by one order of magnitude. A reduction in casting temperature onthe order of about 10° C. is also imposed. In a more general sense, thedwell time should be kept below 0.5 seconds, and preferably below 0.25seconds, and is desirably the minimum that can be achieved with theconstraints of the casting equipment employed.

The casting dispersion should, as noted in the Tables, have an opticaldensity of from about 0.5 to 1, depending on the pore diameter sought;generally, higher optical densities produce higher pore diameters.

The casting dispersion is ordinarily cast onto a moving support by meansof a doctor blade with a knife gap of typically about 250 to 450micrometers, often about 300 micrometers; after the quench, the membraneproduced is typically about 85 to 105 micrometers in thickness forultraporous membranes, and about 105 to about 145 for microporousmembranes. The values may be increased or decreased as desired, as iswell known in the art. While as described, the procedure produces flatsheet membrane, the present invention is equally applicable to castinghollow fiber membrane, and will indeed facilitate the dispersion castingoperation by reducing or even eliminating the atmospheric exposure timepreviously thought to be necessary to such procedures.

In application of the present invention to the casting of hollow fiber,the casting dispersion is spin cast through a hollow die rather thanbeing cast onto a support in a flat film form. The lumen of the hollowfiber is sometimes formed by air or inert gas and the outside quenchedin non-solvent liquid, but usually the quench liquid flows through thecenter and skins the Lumen. If the lumen is formed by a gas, it shouldbe introduced as close to the quench bath as possible. As those familiarwith hollow fiber casting are aware, the die may be immersed in thequench bath in some cases, reducing the atmospheric dwell time of theouter surface of the cast membrane to zero. It is possible, of course,to provide a hollow fiber with both inner and outer skins by employingthe quench liquid in both the lumen and the quench bath.

The cast dispersion is passed into a quench bath, most commonly ofwater, frequently at or near the casting temperature. In the bath, thequench operation precipitates the polymer to produce a skin having therequisite pore sizes, and a support region having the characteristicspinodal structure with a high degree of asymmetry, increasing from theregion immediately membrane is ordinarily washed to free it of entrainedsolvent, and may be dried to expel additional increments of solvent,diluent, and quench liquid, and thus recover the membrane.

The casting operation is amenable to a wide variety of known variations,familiar to those of ordinary skill in the art, as discussed by Wrasidloand others in the prior art. So long as the criteria defined for thepresent invention are met, none of these are excluded.

The resulting membrane produced by the process of the present inventionshares a number of characteristics in common with those taught byWrasidlo and used in commerce in the practice of his technology. Thereare substantial differences which are attained, however.

It is a very great advantage of the improvements provided by the presentinvention that the membrane is produced substantially substantially freeof the artifacts of discontinuous dispersion in the casting medium. Todate, only negligible numbers of polymer spheres have been observed inmembranes cast by the method of the present invention. Compare themembrane of the present invention shown in FIG. 8 with FIG. 7, showingthe prior art problem, which is now largely resolved. The requirementfor the removal of such materials as a part of the membrane washoperations is now eliminated or greatly reduced, reducing the time ofthe wash, the amount of water or other wash constituents is reducedsubstantially, and production of finished product are simplified.

It is another significant achievement of the present invention that thepore diameter of the skin pores is far more consistent, as shown bybubble point testing. Testing of membranes of the present inventionshows that the bubble points desired are achieved far more readily andconsistently at all points in the casting operation, from start-up toconclusion, with markedly reduced standard deviation in pore diameterfor all pore sizes. Pore diameter is the primary quality controlparameter in such membranes, and bubble point is the convenientparameter for defining microporous membrane integrity, the ultimatecriteria being bacterial or microsphere challenge tests.

Indeed, for most pore diameters, it is now possible to maintainproduction at bubble point measurements having a standard deviation ofless than 3, compared to a historical value of about 5 or more for suchmembranes, although in smaller pore sizes it may be necessary to accepta slightly higher standard deviation of less than 5, compared to ahistorical value of 9 or more for comparable membranes. Covariance inbubble point is less than 8, and ordinarily and preferably less than 5,compared to historic values of 9 or higher, and most often above 11 oreven higher, as illustrated in FIG. 5. The skin pores are illustrated inFIG. 2, revealing both a high population of pores, a large proportion ofwhich are at or near the effective controlling pore diameter.

Another feature of the membranes of the present invention is thematerial increase in flow rates for a given controlling pore size. Whilethe relationships and physical features of the membrane of the presentinvention which determine flow rates have not been fully explored asyet, the data show a substantial increase in flow rate as a function ofpore diameter (or pore radius, as discussed in Wrasidlo). The datasuggest an increase in the total number of pores produced in themembrane skin, and possibly a narrower distribution of pore diameter,with few pores having a diameter materially less than the controllingpore diameter as determined by bubble point measurements. See FIG. 2.

What the data do show with certainty is represented in the data plottedin FIG. 3, which demonstrates flow rate plotted against pore diameter,for the membranes of the examples provided in the present application,including the comparative examples. It is apparent that at a given porediameter, the flow rate is materially increased in the presentinvention, when compared to the historical values achieved by theWrasidlo teachings.

In addition, the flow rates are less susceptible to variation duringmanufacture of the membrane, as reflected by a standard deviation inflow rate of normally about 120 or less, more often about 100 or less,and less than 75 except for the largest pore diameters. This improvedconsistency at the higher flow rates is indicative of a material changein the utility of these membranes for the user, and enables the presentinvention to assure higher levels of quality assurance to users,particularly those with critical applications for such membranes. Asshown in FIG. 6, covariance of flow rates of the membranes made by thenew procedures of the present invention are less than 6, and for mostpore diameters, less than 5. FIG. 6 also demonstrates that flow ratecovariance of membranes made by the Wrasidlo procedure has historicallybeen greater than 12.5. Standard deviations are materially reduced aswell.

The consistency achieved in the present invention is a per se benefit inthe production and use of the membranes of the present invention, andrepresents a very substantial gain in productivity and the reduction ofscrap or out-of specification materials. The occurrence of scrap hasbeen reduced to a level consistently less than 5% of production, andlong production runs with no losses to scrap are now frequentlyattained. The improved consistency is also of compelling import to theintegrity of the membrane offered to users.

As shown in FIG. 4, the improved consistency in pore diameter and inflow rate are accompanied by a material increase in the flow rate foreach pore diameter, ranging from an increase in flow rate of from 110ml/min. at a bubble point of about 65, up to about 500 ml/min. at abubble point of about 30, representing a gain in flow rate of 10 to 20percent at a given pore diameter.

A photomicrograph showing the characteristic spinodal structure of themembranes of the present invention is shown in FIG. 1. As those ofordinary skill in the art will recognize, the structure is that producedby spinodal decomposition of the metastable dispersion in the membranecasting operation, and the figure illustrates the significant asymmetrywhich makes such membranes highly effective as depth filters, affordingthe gradual change in the apertures through the support region such thatthe changing effective pore diameter is progressive.

The skin of the membrane is quite thin, and difficult to delineateprecisely by photomicroscopy in cross section. Where skin pores aredirectly observable, as shown in FIG. 2, the number of pores and theirgeneral regularity is directly observable.

A number of membranes have been made and tested in accordance with thepresent invention, and compared with the commercially availableembodiments of the Wrasidlo technology. These efforts and comparisonsare set out in the following examples.

SPECIFIC EXAMPLES

In the following examples, the formulations employed are those set forthin Table I and Table II, above. The conditions of casting are also thoseset forth in the tables. Both the examples of the present invention andthose based on the Wrasidlo teachings were performed on the sameequipment, and under the same conditions; the only differences are thoseset forth in the above Tables. As inspection will show, no adjustmentsof the formulations to achieve specifically targeted pore diameters weremade in most cases, with the results that some of the bubble pointvalues are higher than desired. Further adjustments to achieve thedesired bubble point, reflecting a specifically required pore diameterare know to the art, and are specifically taught by Wrasidlo.

A plurality of rolls of membrane were cast. Each roll was sampled at aplurality of predetermined locations, across the web and throughout thelength of the cast membrane. The values for each sample were averaged,and the standard deviation determined, for both bubble point and flowrate. The results obtained are shown in the following Table III:

                  TABLE III                                                       ______________________________________                                        Example                                                                              Flow    σ BP   σ                                                                            BRK  σ                                                                            ELG  σ                       ______________________________________                                        30-1   2393    113     38   0.8  548  21   26   3                             30-2   2495    100     38   0.6  535  14   30   2                             30-3   2340    73      36   1.6  544   5   30   3                             30-4   2311    45      37   1.1  518  16   27   2                             30-5   2097    73      38   0.7  578  14   30   1                             30-6   2337    184     38   2.6  607   9   29   2                             30-7   2232    98      38   1.4  607  23   26   3                             30-8   2402    74      36   2.1  600  23   24   4                             30-9   2470    77      36   1.3  558  10   24   2                             30-10  2586    75      35   2.3  569   8   24   2                             30-AVE 2366      91.2  37    1.45                                                                              566  14.3 27     2.4                         45-1   1663    110     45   3.1  552  20   29   3                             45-2   1776    83      44   2.9  536   8   29   3                             45-3   1495    50      49   3.5  521  13   31   2                             45-4   1525    38      47   2.1  523   8   29   5                             45-5   1635    53      45   0.8  538   9   32   3                             45-6   1671    42      44   1.6  525  11   29   2                             45-7   1705    56      44   2.2  530  22   29   3                             45-8   1720    75      44   1.3  529   7   36   1                             45-9   1742    73      44   3.3  520  19   34   5                             45-10  1778    59      42   2.2  504  11   28   2                             45-11  1673    114     46   1.4  556  14   25   3                             45-12  1844    79      43   3.2  548  10   34   4                             45-13  1737    118     47   1.5  554  24   30   5                             45-14  1805    52      46   2.0  561   8   33   2                             45-15  1697    76      48   1.4  554  15   32   3                             45-16  1761    87      48   3.0  528   9   29   2                             45-17  1608    76      48   2.5  584  12   34   2                             45-18  1779    44      46   3.6  569  15   32   2                             45-19  1556    53      48   1.7  545  16   32   3                             45-20  1698    32      45   1.5  547  16   32   3                             45-21  1626    95      48   1.4  513  18   32   3                             45-22  1802    43      44   1.7  488  18   30   3                             45-23  1657    123     42   3.2  568   6   31   2                             45-24  1747    54      45   3.2  561  17   31   4                             45-25  1534    108     48   1.6  578  19   29   3                             45-26  1630    84      46   2.5  561  17   29   4                             45-27  1634    70      47   2.2  572  13   27   3                             45-28  1787    57      44   1.8  558  17   29   3                             45-AVE 1689      71.6  45.6 2.2  544  14   31     3.0                         55-1   1234    45      57   3.0  574  23   36   5                             55-2   1296    64      64   1.8  573  14   30   3                             55-3   1192    47      59   3.0  612  16   34   4                             55-4   1228    66      58   2.0  595   9   34   2                             55-5   1359    101     54   2.9  619  11   34   2                             55-6   1418    26      52   2.4  598  20   36   4                             55-7   1271    97      56   4.0  600  13   39   3                             55-8   1319    55      57   2.7  588  16   38   2                             55-9   1284    76      58   4.2  613  25   35   6                             55-10  1364    57      57   2.5  609  25   35   4                             55-11  1561    97      48   2.3  523  20   29   5                             55-12  1598    66      49   2.3  555  17   32   4                             55-13  1258    103     56   2.8  553  11   39   4                             55-14  1439    36      53   1.4  579  15   35   4                             55-15  1253    87      56   3.0  533  23   28   4                             55-16  1363    67      54   1.0  573  13   35   3                             55-17  1387    39      56   2.3  615  12   38   2                             55-18  1430    60      55   3.2  586  20   37   4                             55-19  1300    47      56   1.7  520  23   34   6                             55-20  1410    49      56   1.5  548  25   39   3                             55-21  1290    61      55   3.5  571  11   33   4                             55-22  1418    60      54   0.9  590  26   35   4                             55-AVE 1349      63.9  55.5  2.47                                                                              578.5                                                                              17.6   34.8                                                                               3.7                         65-1   1146    50      60   2.5  676  22   39   4                             65-2   1403    22      60   2.4  665  24   40   4                             65-3   1108    57      59   3.7  648  17   44   4                             65-4   1115    36      62   1.5  637  26   36   5                             65-5   1060    65      60   3.6  613  29   34   5                             65-6   1155    40      60   2.4  610  21   33   3                             65-7   1013    46      64   4.4  666  19   35   2                             65-8   1037    30      68   2.8  697  20   36   3                             65-9    859    93      72   12.0 664  31   39   2                             65-10   854    77      78   8.4  648  24   36   3                             65-11   986    76      64   7.6  614  35   35   7                             65-12   931    82      73   7.5  616  37   36   5                             65-13   949    61      65   6.4  713  27   33   4                             65-14  1026    75      62   6.9  694  25   33   4                             65-15  1046    79      59   2.1  695  24   35   5                             65-16  1095    47      57   2.5  684  23   34   4                             65-17  1217    94      55   5.3  642  12   38   4                             65-18  1346    50      52   4.8  638  23   38   3                             65-19  1019    37      61   2.1  657  12   37   2                             65-20  1146    59      58   1.7  658  18   38   3                             65-21  1017    50      63   3.4  674  14   36   4                             65-22  1101    40      60   3.4  688  18   38   4                             65-23   932    83      72   9.4  668   9   34   2                             65-24  1041    46      63   5.8  630  17   32   3                             65-AVE 1058    58      62.8 4.7  658  22.0   36.2                                                                               3.7                         ______________________________________                                    

In the foregoing data in Table III, each Example represents a roll ofmembrane, made as described above, bubble point (BP) is reported as thearithmetic mean of all quality control samples taken for each roll. Thebubble point represents the pressure, in psi, of breakthrough of airapplied to a 90 mm disc sample wetted with distilled water, common tothe membrane art. Flow is reported as the arithmetic mean of all samplesfor each roll and represents the flow of distilled water, in ml perminute, passing through a 90 mm disc at an applied pressure of 10 psi.BRK represents tensile strength at break in grams, while ELG iselongation at break, in percent.

                  TABLE IV                                                        ______________________________________                                        COMPARATIVE EXAMPLES                                                          TYPE    Flow    σ                                                                              BP   σ                                                                            BRK  σ                                                                            ELG  σ                       ______________________________________                                        X65-AVE  972    129.8  63.1 9.3  588  24.6 29.0 3.2                           X55-AVE 1278    181.9  51.0 6.9  512  19.8 24.7 2.2                           X45-AVE 2059    403.0  33.8 4.4  491  35.7 23.7 3.1                           X30-AVE 2518    322.6  29.2 2.6  467  26.0 21.8 2.4                           X25-AVE 3558    481.0  22.8 2.6  469  22.8 23.3 2.3                           ______________________________________                                    

In Table IV, the values reported for each comparative example, thevalues reported are the arithmetic mean of all values for actualcommercial production of a substantial number of rolls of membrane,produced as indicated in Table I, under the same conditions as thoseemployed in the present invention, except as noted above.

The principles, preferred embodiments, and modes of operation of thepresent invention have been described in the forgoing specification andexamples. The invention is not intended to be constrained thereby, orconstrued as limited to the particular forms disclosed, since these areintended to be illustrative rather than restrictive. Variations andchanges may be made by those skilled in the art without departing fromthe spirit and scope of the present invention as defined in thefollowing claims.

What is claimed is:
 1. A method of making a highly asymmetric integrallyskinned polymer membrane, said method comprising:A. mixing a polymer, asolvent and a non-solvent to produce a metastable liquid-liquiddispersion within the binodal or spinodal at a casting temperature, andhaving an optical density of from about 0.5 to about 1, B. casting saiddispersion into a thin layer at said casting temperature, C. passingsaid cast layer within a time of less than 0.5 seconds at said castingtemperature into a solvent extraction quench bath of non-solvent quenchliquid in which said solvent is freely miscible and in which saidpolymer is substantially insoluble, and effecting precipitation of amajor proportion of said polymer by spinodal decomposition, and D.recovering from said quench bath a membrane.
 2. The method of claim 1wherein said time is less than about 0.25 seconds.
 3. The method ofclaim 1 wherein said time is not more than about 0.1 seconds.
 4. Amembrane product which is the product of the process of claim 1.